TEMPORAL VARIATION IN HUMORAL AND CELL-MEDIATED IMMUNE-RESPONSE IN A SIGMODON HISPIDUS POPULATION

We examined the hypothesis that environmentally induced alteration of immunocompetence is a primary physiological mechanism by which population survival is regulated. The involvement of disease and immunocompetence in the regulation of small mammal populations is not well understood but a strong relationship between physiological stress and immunity has been demonstrated. Its involvement is further supported by several studies, which have provided evidence that survival, incidence of disease, and selected gross measures of immunity change temporally with density and season. We examined this hypothesis by documenting the existence of temporal alterations in selected measures of both humoral and cell-mediated immune function in a population of cotton rats (Sigmodon hispidus), a species known to fluctuate widely in density, inhabiting a tallgrass prairie in central Oklahoma. A total of 310 cotton rats was collected from August 1989 to May 1991 (10 collections) using conventional live-trap methods. In addition to morphological characteristics of primary and secondary lymphoid organs, the ability of splenocytes to produce specific antibody in response to a single injection of sheep erythrocytes and to proliferate in response to in vitro exposure to mitogens was measured to assess immunocompetence. Whole blood was collected for the determination of total peripheral blood leukocyte counts, packed cell volume, and total serum proteins. Statistically significant differences in all variables except total serum protein existed among the 10 collections and most of this variability was independent of specific seasonal time periods. Bimodal rhythmicity was characteristic of temporal changes for several measures of immunocompetence. Changes over time in the average immunocompetence of this population could have reflected a shift in genotypic polymorphism; several other plausible explanations are offered to explain the observed temporal patterns.